Automatic gain control system



Feb. 17, 1942. J, B, MQOREY 2,273,132

AUTOMATIC GAIN CONTROL SYSTEM Filed Dec. 6, 1940 slk Q ge Nl Q slk- @E- 1 l@ @f @L b gm l H11 w I j Q.- //N R4, ggV f gg 'A1-'TORNEY Patented Feb. 17, 1.942

AUTOMATIC GAIN CONTROL SYSTEM John B. Moore, Riverhead, N. Y., assignor to Radio (LOrporationA of America, a corporation of Delaware Application December 6, 1940, Serial No. 368,818

(Cl. Z50-20) 4 Claims.

My present invention relates to automatic gain control systems, and more particularly to an improved vautomatic gain control circuit for a heterodyne receiver of the short wave type.

In a short wave receiver of the heterody-ne type intended for use in commercial service (e. g., the 3 to 24 megacycle band), the radio frequency amplifier network must generally comprise two or more stages of tuned amplification. This is not so much to provide high amplificationy as it is to provide the required selectivity prior to the first converter. Another reason for utilizing the plurality of stages of tuned radio vfrequency amplification is to isolate the local oscillator from the signal collector (antenna). The latter is of great importance in cases Where a number of short wave receivers may have to be operated from a common antenna, or from a plurality of closely grouped antennae.

The screen grid amplifier tubes normally einployed in such cascaded radio frequency amplifier stages, in connection with their associated tuned circuits, may provide more amplification than is required. Where automatic gain control is applied to three such cascaded radio frequency amplifier stages, there may result Aa condition such as the following. The receiver is tuned to a weak signal of only 20 micro-volts; that is, there is applied to the input terminals of the receiver a signal of approximately 20 microvolts. On a channel not differing greatly in frequency from that of the desired signal, there is a strong signal which is delivering some 20,000 micro-volts to the input terminals of the same receiver. Since the receiver is tuned to a weak signal, automatic gain `control maintains a relatively high gain in the radio frequency amplifier tubes. The result is that the 20,000 micro-Volt signal on the nearby channel will seriously overload the first converter stage.

To prevent such overloading, the radio frequency gain prior to the first converter stage must be limited to some predetermined maximum value. At the same time, however, it is necessary to provide a certain minimum value of radio frequency gain to retain optimum signal to noise ratio on weak signals. It may, therefore, be stated that it is one of the main objects of the present invention to provide improved performance of a heterodyne, or double heterodyne, type of short wave receiver by:increasing the protection against spurious responses; providing a more nearly constant gain throughout the radio frequency amplifier tuning range; minimizing the possibility of overloading the earlier stages of the receiver; and the simultaneous retention of a low noise equivalent so that` the signal to noise ratio on a signal of any strength will not be appreciably impaired by the aforementioned features. n

. network. Such a system is subject to overloading. Hence, it is another important object of this invention to provide a radio receiver of the short wave type wherein the tube in the first radio frequency amplifier stage has applied to its control grid the automatic gain control voltage that is supplied to the intermediate frequency stages which succeed the first converter network, while the radio frequency amplifier tubes following the controlled amplifier are maintained at a fixed predetermined gain.

Other objects of this invention are to provide at least three gain-controlled stages in a short Wave receiver of the heterodyne type so as to secure proper gain control performance; to provide moderate amplification ahead of the first converter to insure that the noise level on weak signals will be determined by the first amplier stage and not by the first converter; to provide the required additional gain in the intermediate frequency amplifier stages following the first converter and to control such stages; to provide full radio frequency amplier gain on weak signals thereby to retain the optimum signal to noise ratio and to make the gain control network reduce first the intermediate frequency amplifier gain and later the radio frequency amplier gain as the strength of the input signal is increased; the latter action being secured by operating the controlled radio frequency amplifier tube at a higher screen grid voltage than are the tubes in the controlled intermediate frequency amplifier stages.

Still other objects of the invention are to provide a short wave receiver of the superhetero- Z dyne type wherein there is employed a plurality of cascaded tunable radio frequency amplifier stages and a plurality of intermediate frequency amplier stages, and an automatic gain control circuit controlling one or more of the intermediate frequency amplifier stages and solely the screen voltages in the -controlled intermediate amplier stages employing the same type of tube.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in l connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Fig. 1 shows a circuit diagram of the receiving system embodying the invention,

Fig. 2 graphically illustrates the effect of using the present invention.

Referring now to Fig. 1, there is shown a circuit diagram of a short wave receiver of the type used in diversity reception in the 3-24 megacycle commercial service band. The signal collector I is a grounded antenna circuit, a dipole, or a radio frequency distribution line. The collected signals are impressed on the tunable signal input circuit 2 of the first one of the radio frequency amplifier tubes 3. The description for the stage including tube 3 should be understood as being applicable to the remainder of the tunable amplifiers, save in the matter of control for gain.

The numerals and 6 designate the tunable input circuits of the following amplifiers 3' and 3" respectively. Each of the latter are tuned to a common modulated signal carrier frequency. The first converter 8 may also be equipped with a tunable signal input circuit (not shown) similar to the prior circuit 6. The radio frequency heterodyne oscillator 9 is schematically represented, and may be provided with a tunable oscillator tank circuit (not shown) which is adjusted over a range of oscillation frequencies differing constantly from the selected signal frequency range by the operating intermediate frequency of 450 kilocycles (kc.).

The first intermediate frequency amplifiers embody tubes I2 and I3 arranged in cascade. Tube I 2 is of the same screen grid type as tube 3. The signal input circuit II of tube I2 is reactively coupled to circuit I0. The transformer I4 couples the output electrodes of tube I2 to the input electrodes of tube I3. It may be assumed that the same type of amplifier tube is employed at each of the stages up to stage I3. The screen grids of tubes I2 and I3 are connected by common lead I5 to a desired positive potential point on the direct current voltage supply potentiometer I E. The screen grid of tube 3 is connected by lead I1 to a point on the potentiometer IG which is of substantially higher potential than the point to which the screens of tubes I2 and I3 are connected. Of course, the potentiometer I6 may supply the +B voltages to the plates of the various amplifier tubes. 'Ihe intermediate coupling transformers I0', III and I8 consist in each case of coupled tuned circuits, and each of the tuned circuits is generally shunted by a damping resistor. Each transformer is adjusted to pass the desired width of band centering on the desired mid-band frequency of 450 kilocycles.

The signal energy output of transformer I8 is applied to a second converter. The latter is fed with oscillations of 400 kilocycles from oscillator 40. There is produced in the output of the converter 4I intermediate frequency energy of a 50 kilocycle frequency. The second intermediate frequency amplifier may comprise stages 42 and 43 which may be constructed in the same manner as the first intermediate frequency amplifier stages, except for gain control and tuning. The amplified second intermediate frequency energy may be applied to a rectifier 50. The latter may be of the diode type, and will provide audio voltage for the succeeding audio utilization circuits. The same rectifier or a separate rectifier, may

also derive the direct current voltage required for gain control from the second intermediate frequency energy. Those skilled in the art are fully acquainted with the manner of connecting the AGC line 2U to the anode end of the rectifier load resistor.

The circuit details of the rectifier 50 need not be shown since they are very well known to those skilled in the art. Across the rectifier output load resistor is produced a direct current Voltage which varies directly in magnitude with the carrier amplitude. The voltage` is applied over the AGC lead 20 through filter 2I which functions to suppress modulation components in the rectified voltage output of rectifier 50. The lead 20 is connected to the signal grid of amplifier 3 through radio frequency filter choke coil 22 and through the coil of input circuit 2. The gain of each of stages 3' and 3" is not varied; they remain at a predetermined gain.

The lead 2U is connected to the low potential ends of the input coils of each tubes I2 and I3. Each gain control lead to tubes I2 and I3 includes a filter choke coil similar to 22. Hence, the first of the cascaded radio frequency amplifiers and each of the intermediate amplifiers are varied in gain as the carrier amplitude varies. Since the AGC voltage is applied in an increasingly negative sense as signal carrier amplitude increases, it will be seen that the carrier amplitude at the input of rectifier 50 tends to be maintained constant despite wide variations in carrier amplitude at the collector I.

The AGC network acts to reduce the gain of each of tubes I2 and I3 up to a predetermined value of carrier amplitude increase, while for amplitudes above that value the gain of tube 3 is reduced in addition. This action is secured by virtue of the differential biasing of the screen grids of tubes 3 and I2-I3. By making the screen of tube 3 of higher positive potential, the gain of tube 3 holds up longer than does that of tubes I2 and I3. The AGC action, then, is delayed on tube 3. The action of the gain control circuit will be better understood with the following illustrative values. It will be understood that a negative bias source 60 may be included in the AGC lead to the grids. This provides normal, no-signal gain. Assume a noise equivalent of 0.5 micro-volt at the receiver input terminals, and of 5 micro-volts at the signal grid of converter 8. To insure that the final signal to noise ratio will be determined by the 0.5 micro-volt noise equivalent at the input terminals, there must be provided a total voltage gain, that is from the input terminals to the converter signal grid, of not less than 30. This amplifies the various noise components prior to the converter up to 15 micro-volts at the converter signal grid. The converter noise equivalent of 5 micro-volts, when added to this 15 micro-volts by rms addition, gives a total of about 15.8 micro-volts noise effective at the converter signal grid. The converter, therefore, adds very little to the total noise output of the receiver.

If it is now assumed that a signal to noise ratio of 40 decibels (100:1) is desirable and quite satisfactory, the conclusion is reached that all input signals of from zero up to times 0.5 micro-volt, or 50 micro-volts, should be amplified ,the full 30-fold. For higher signal inputs the radio frequency amplifier gain may be slowly reduced. The result of this is that the input signal to noise ratio is maintained up to a signal input of 50 micro-volts. As the input voltage is further increased the radio frequency amplifier gain at tube 3 is slowly reduced by the AGC action. The net result is that increases of signal input above the 50 micro-volts Value produce an over-all improvement in signal to noise ratio which is less than the improvement at the input terminals of the receiver. This reduction is, however, of no consequence since it takes place progressively, and only on signal to noise ratios already better than the assumed satisfactory Value of 100: 1 (40 decibels).

In Fig. 2 there is shown graphically the foregoing explanatory matter. In Fig. 2 there is plotted Noise level in decibels below 100% modulation against Carrier input in micro-volts. The full line curve which levels off at the higher values of carrier input is due to the AGC action of this invention. The straight, descending dashed line is the type of characteristic which would be obtained, assuming no overloading, with fixed gain. If the controlled amplifier 3 were operated at the same screen grid voltage as that of the controlled intermediate frequency amplifiers l 2 and I3, the resultant characteristic would be as shown by the dotted line lying above the solid curve.

It will, therefore, be observed that with the present AGC system, there is provided at least three controlled stages so as to secure proper control performance. Furthermore, there is provided moderate amplification prior to the converter 3 to insure that the noise level on weak signals will be determined by the first stage 3 and the input circuit (0.5 micro-volt noise equivalent) and not by the converter 8 (5 micro-volts Y noise equivalent). In addition, the present invention provides additional gain in the intermediate frequency amplifier networks with gain control therefor, and there is provided full radio frequency amplifier gain on weak signals to retain the optimum signal to noise ratio.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a radio receiver provided with a plurality of cascaded tuned radio frequency amplifier stages, a converter stage and a plurality of intermediate frequency amplifiers; the improvement which comprises means responsive to carrier amplitude variations for controlling the gain of solely the first of the cascaded radio frequency amplifiers and each of the intermediate frequency amplifiers, said one amplifier stage and said intermediate frequency amplifiers each including an amplifier tube of the same screen grid type, and means for establishing the screen grid electrode of said one radio frequency amplifier stage at a sufficiently higher positive voltage than the`screen grids of said intermediate frequency amplifiers to prevent reduction of the gain of said first amplifier until the gain of said intermediate amplifier has been reduced to a predetermined extent.

2. In a short wave receiver of the type comprising at least two tuned radio frequency amplier tubes arranged in cascade, a converter network and at least two intermediate frequency amplifier tubes; the improvement which comprises at least a first of said cascaded tubes being of the screen grid type, said intermediate frequency amplifiers each comprising a tube of the same screen grid type, means for establishing the screen grid electrode of the first tube at a substantially higher positive potential than the screen grids of the intermediate frequency amplifier tubes, an automatic gain control network responsive to carrier amplitude increase above a predetermined amplitude for varying solely the gain of each of said intermediate frequency amplifiers, and a connection from said gain control to said first amplifier tube whereby the gain of said first tube is automatically reduced simultaneously with the gain reduction of the intermediate frequency amplifier tubes in response to carrier amplitude increase above a higher predetermined value and said higher positive potential being of suiiicient value to prevent gain reduction of said first tube until said predetermined higher value of carrier amplitude is atta-ined.

3. In a short wave receiver of the type comprising at least two tuned radio frequency amplifier tubes arranged in cascade and each tuned to a carrier frequency in the 3-24 megacycle range, a converted network and at least two intermediate frequency amplifier tubes; the improvement which comprises the first of said cascades tubes being of the screen grid type, each of said intermediate frequency amplifiers comprising a tube of the same screen grid type, means for establishing the screen grid electrode of the first tube at a substantially higher positive potential than the screen grids of the intermediate frequency amplifier tubes, an automatic gain control network responsiveto carrier amplitude increase above a predetermined amplitude for varying the gain of each of said intermediate frequency amplifiers, and a connection from said gain control to said first amplifier tube whereby the gain of said rst tube is automatically reduced simultaneously with the gain reduction of the intermediate frequency amplifier tubes in response to carrier amplitude increase above a higher predetermined value and said higher positive potential being of sufficient value to prevent gain reduction of said first tube until said predetermined higher value of carrier amplitude is attained.

4. In a radio receiver provided with a plurality of cascaded tuned radio frequency amplifiers, a converter stage and a plurality of intermediate frequency amplifiers; the improvement which comprises a signal carrier rectifier means, responsive to carrier amplitude variation, for controlling the gain of solely the first of the cascaded radio frequency amplifiers and each of the intermediate frequency amplifiers, and additional means for delaying the action of said responsive means on said first radio frequency amplifier until subsequent to a predetermined control of said intermediate frequency amplifiers, said additional means comprising said one amplifier and said intermediate frequency amplifiers each including an amplifier tube of the screen grid type, and means for establishing the screen grid electrode of said one radio frequency amplifier at a suiiiciently higher positive voltage than the screen grids of said intermediate frequency amplifiers to provide said delay action on said first amplifier.

JOHN B. MOORE. 

